\Header{validObject}{Test the Validity of an Object}
\alias{setValidity}{validObject}
\keyword{programming}{validObject}
\keyword{classes}{validObject}
\begin{Description}\relax
The validity of \code{object} related to its class definition is
tested.  If the object is valid, \code{TRUE} is returned; otherwise,
either a vector of strings describing validity failures is returned,
or an error is generated (according to whether \code{test} is
\code{TRUE}).

The function \code{setValidity}  sets
the validity method of a class (but more normally, this method will
be supplied as the \code{validity} argument to \code{\Link{setClass}}).  The method should be a function of one
object that returns \code{TRUE} or a description of the non-validity.\end{Description}
\begin{Usage}
\begin{verbatim}
validObject(object, test)

 setValidity(Class, method, where = topenv(parent.frame()) )
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{object}] Any object, but not much will happen unless the
object's class has a formal definition.
\item[\code{test}] If \code{test} is \code{TRUE}, and validity fails the
function returns a vector of strings describing the problems.  If
\code{test} is \code{FALSE} (the default) validity failure generates
an error.
\item[\code{Class}] the name or class definition of the class whose validity
method is to be set.
\item[\code{method}] a validity method;  that is, either \code{NULL} or a
function of one argument (the \code{object}).  Like
\code{validObject}, the function should return \code{TRUE} if the
object is valid, and one or more descriptive strings if any problems
are found.  Unlike \code{validObject}, it should never generate an
error.

\item[\code{where}] the modified class definition will be stored in this
environment.
\end{ldescription}


Note that validity methods do not have to check validity of any
slots or superclasses:  the logic of \code{validObject} ensures
these tests are done once only.  As a consequence, if one validity
method wants to use another, it should extract and call the method
from the other definition of the other class by calling
\code{\Link{getValidity}}:  it should \emph{not} call
\code{validObject}.\end{Arguments}
\begin{Details}\relax
Validity testing takes place \dQuote{bottom up}:  first the validity
of the object's slots, if any, is tested.  Then for each of the
classes that this class extends (the \dQuote{superclasses}), the
explicit validity method of that class is called, if one exists.
Finally, the validity method of \code{object}'s class is called, if
there is one.

Testing generally stops at the first stage of finding an error, except
that all the slots will be examined even if a slot has failed its
validity test.\end{Details}
\begin{Value}
\code{validObject} returns \code{TRUE} if the object is valid.
Otherwise a vector of strings describing problems found, except that
if \code{test} is \code{FALSE}, validity failure generates an error,
with the corresponding strings in the error message.\end{Value}
\begin{References}\relax
The R package \pkg{methods} implements, with a few exceptions, the
programming interface for classes and methods in the book
\emph{Programming with Data} (John M. Chambers, Springer, 1998), in
particular sections 1.6, 2.7, 2.8, and chapters 7 and 8.

While the programming interface for the \pkg{methods} package follows
the reference, the R software is an original implementation, so
details in the reference that reflect the S4 implementation may appear
differently in R.  Also, there are extensions to the programming
interface developed more recently than the reference.  For a
discussion of details and ongoing development, see the web page
\url{http://developer.r-project.org/methodsPackage.html} and the
pointers from that page.\end{References}
\begin{SeeAlso}\relax
\code{\Link{setClass}}.\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
setClass("track",
          representation(x="numeric", y = "numeric"))
t1 <- new("track", x=1:10, y=sort(rnorm(10)))
## A valid "track" object has the same number of x, y values
validTrackObject <- function(x){
    if(length(x@x) == length(x@y)) TRUE
    else paste("Unequal x,y lengths: ", length(x@x), ", ", length(x@y),
    sep="")
}
## assign the function as the validity method for the class
setValidity("track", validTrackObject)
## t1 should be a valid "track" object
validObject(t1)
## Now we do something bad
t1@x <- 1:20
## This should generate an error
## Don't run: try(validObject(t1))

\end{ExampleCode}
\end{Examples}

